5 Crucial Steps to Clean Your Home’s Flooded Walls

5 Crucial Steps to Clean Your Home’s Flooded Walls
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You can’t easily build a “floodproof” wall,  but you can greatly improve the washability of a home’s walls after a flood event.

I stopped the other day to help a woman whose home had been flooded. She was struggling to roll a replacement refrigerator into her gutted, flooded home.

I noticed as I went in the house that she had exposed spray foam in the wall cavities of her stick-framed home, but it was showing signs of black mold growth in every crevice and cranny. Dense cell spray foam is not supposed to mold.

So what happened? Heavy spray foam (2 lbs per cubic foot density) is considered non-porous. The short answer is, nature brought its own fuel supply. To be clear, it’s not the foam that’s growing mold. It’s the nasty bacteria and mold spores left behind by floodwaters. The wall was not adequately cleaned and sterilized, so mold moved in.

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Do You Need to Open the Walls?

There’s no such thing as a flood that does not require you to remove the interior wallboard. Even if your walls were made of stainless steel, you’d have to pull off the drywall or paneling and wash the steel, or end up with creeping mold growth.

This is true of wood frame homes, and it’s also true of concrete block (or concrete masonry) homes. Both are porous materials. You have, at best, about 2 days before the mold spores and pathogen begin to creep into your structure. After that, your home may need a deeper clean, with harsher chemicals and professional gear if you want to save it.

So take it step by step. 

1. Shut off the Power. Chances are your electrical outlets were submerged. Don’t try to save them. They will rapidly corrode and become fire hazards if you leave them as they are. Cut them off and cap the wires. I recommend fast clips, to make installing new ones easier later.

2. Tear Out or Cut Off Walls. Whether you have paneling or drywall, you need to cut it off far enough above the flood level to avoid any soaked materials. Do not try to save the cut off paneling, unless you had coated it previously on all four sides with acrylic paint. The risk of contamination is not worth the few dollars you save. When you replace this wallboard later, if you expect future flooding, consider installing wainscoting that has been coated with a water-resistant epoxy on all sides (such as a marine epoxy), or non-porous sheet products such as HDPE plastic, although these are quite expensive, especially with shipping. 

3. Insulation Exorcism. First, if you have any kind of porous insulation, such as fiberglass or cellulose, all of it needs to come out and be disposed of. But other types of insulation below the flood level are problematic too. Rigid foam board is mostly non-porous, but can trap water against the house sheathing. 

My recommendation, if you want to save it, would be to cut it off, pull it out and clean all four sides, along with the wall cavity where it lives. The only insulation you might leave in the cavity is dense cell spray foam. But as I mentioned before, it needs to be cleaned aggressively, so its rough surface doesn’t become a petri dish for mold.

4. Bleach, then Vinegar, and Rinse. Let’s assume I’ve convinced you to tear all your wall panels off and remove all porous insulation. You may be tempted to spray the surfaces with bleach and call it good. But bleach does not penetrate surfaces well. You need to do a separate cleaning with a 5% or better vinegar solution. Do not mix bleach and vinegar together. You can use a low-cost pumpable garden sprayer to apply these chemicals.

The vinegar will penetrate more deeply, and kill mold spores more effectively. Otherwise, as soon as the house gets warm and moist again, the mold could spring forth in hiding

By the way, if you‘ve seen the 30 percent vinegar lining Home Depot’s shelves for $19.95, don’t feel you have to upgrade unless you want to. I did the math, and if you dilute it to 5% you’re only saving about 17 cents per gallon. 

You don’t need a pressure washer to get this job done. You can do it with a garden hose, or, if your water supply is cut off, fill 5-gallon buckets from the pool. It sounds drastic, but you have limited time before mold gets a foothold.

5. Dry Thoroughly. Turn on every air conditioner in the home, or crank the heat pumps down to about 61 degrees. Leave them on continuously for 2 or 3 weeks. These will remove moisture from the wall studs or concrete blocks. Keep in mind that many concrete homes have hollow, non-reinforced block walls. During a flood, these can fill with water, and take a long time to dry out. 

Try to get the household humidity level stable at 50 percent or lower. One caveat to the masonry moisture is that concrete acts to some degree as a filter when filled this way, so the blocks are not likely to “wick” additional contaminants into the home. 

Can Walls Be Made More Flood Ready?

Building Science Corp. offers several schematics for flood resilient walls. If you look at these, keep in mind that in order for these schematics to work, you need a perfect installation. For example, they suggest that you might leave the rigid foam insulation in place, if it is fully adhered to the interior of the outer wall. I’m skeptical of that kind of precision.

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This illustration of a flood-resistant wall assembly touches on most of the problem areas. The interior wall covering is a removable wainscot, and the wall cavity, including any framing studs, has been coated with acrylic paint. It’s still not a foolproof wall, however. The point where the rigid foam board touches the underlayment for the brick veneer has to be sealed with 100% accuracy; otherwise it’s likely to allow contaminated water to seep up and grow things. That’s a risky assumption.  


When Is It Ok to Just Dry It Out and Move On?

The quick answer is “never.” I’ve heard from more than one flooded homeowner that they took on water, but were able to dry it out without removing the walls and it seems fine now. There is no reality in which this is a good idea. Flood water contents differ slightly, depending on the source of the immersion, but none are “clean.”

You may have heard reports of “flesh eating bacteria” being found on Florida beaches after recent hurricanes. This is not conspiracy hype, it’s a real thing. And this is just one of the awful things floodwaters could bring into your house. There’s also E. coli from your neighbor’s aging septic, salt from seawater storm surge, and, depending on your location, lawn pesticides, weed killers, and chemicals from industry.

An Australian study of “flood mud” also connected flooding events with pathogens that cause diarrhea, respiratory infections, tetanus, and other unpleasant maladies.

The bottom line is: If flood water touched it, you need to sterilize or toss it. The fact that it’s dry now doesn’t mean it’s not a threat. Both bacteria and mold spores can lie dormant for months or even years. All that’s need to trigger them in some cases is a humidity level above 60 percent. That’s a low bar in the southeast especially, when outdoor humidity often surpasses 90 percent in the summer.

Just because these walls look fine doesn’t mean they’re not seething with mold growth all the way to flood level and beyond. All of these electric outlets need to be pulled, wires re-stripped and new ones installed when the wallboard is removed.